NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c3.

The proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio gigas were examined while varying the pH and the redox potential. The analysis of the NMR reoxidation pattern was based on a model for the electron distribution between the four haems that takes into account haem-haem redox interactions. The intramolecular electron exchange is fast on the NMR time scale (larger than 10(5) s-1). The NMR data concerning the pH dependence of the chemical shift of haem methyl resonances in different oxidation steps and resonance intensities are not compatible with a non-interacting model and can be explained assuming a redox interaction between the haems. A complete analysis at pH* = 7.2 and 9.6, shows that the haem-haem interacting potentials cover a range from -50 mV to +60 mV. The midpoint redox potentials of some of the haems, as well as some of their interacting potentials, are pH-dependent. The physiological relevance of the modulation of the haem midpoint redox potentials by both the pH and the redox potential of the solution is discussed.